The general concept of a discharge device which utilizes a hollow cathode for increased current capability is disclosed in the prior art. A hollow cathode glow discharge utilizing a single, nearly spherical hollow cathode is described by A. D. White in Journal of Applied Physics, Vol. 30, No. 1, May 1959, pp. 711-719. The author reported a stable discharge and negligible deterioration from sputtering. The basic mechanisms contributing to the hollow cathode effect are described by G. Schaefer et al. in Physics and Applications of Pseudosparks, Ed. by M. A. Gundersen and G. Schaefer, Plenum Press, New York, 1990, pp. 55-76. Measurements of the temporal development of hollow cathode discharges are described by M. T. Ngo et al. in IEEE Transactions on Plasma Science, Vol. 18, No. 3, June 1990, pp. 669-676.
A variety of hollow cathode structures for fluorescent lamps have been disclosed in the prior art. A directly-heated hollow cathode having an interior coating of an emissive material is disclosed in U.S. Pat. No. 4,523,125, issued Jun. 11, 1985 to Anderson. A shielded hollow cathode for fluorescent lamps is disclosed in U.S. Pat. No. 4,461,970, issued Jul. 24, 1984 to Anderson. A hollow electrode having an interior coating of an emissive material that varies in thickness is disclosed in U.S. Pat. No. 2,847,605, issued Aug. 12, 1958 to Byer. A short arc fluorescent lamp having hollow cathode assemblies is disclosed in U.S. Pat. No. 4,093,893, issued Jun. 6, 1978 to Anderson. Cup shaped electrodes containing emissive material for use in cold cathode fluorescent lamps are disclosed in U.S. Pat. No. 3,906,271, issued Sep. 16, 1975 to Aptt, Jr., and U.S. Pat. No. 3,969,279, issued Jul. 13, 1976 to Kern. A fluorescent lamp wherein a filament is positioned within a cylindrical shield is disclosed in U.S. Pat. No. 2,549,355, issued Apr. 17, 1951 to Winninghoff. Additional hollow cathode discharge devices are disclosed in U.S. Pat. No. 1,842,215, issued Jan. 19, 1932 to Thomas; U.S. Pat. No. 3,515,932, issued Jun. 2, 1970 to King; U.S. Pat. No. 4,795,942, issued Jan. 3, 1989 to Yamasaki; U.S. Pat. No. 3,390,297, issued Jun. 25, 1968 to Vollmer; and U.S. Pat. No. 3,383,541, issued May 14, 1968 to Ferreira.
An electrical discharge electrode including a plurality of tubes, which are filled with an electron emissive material and embedded in a metallic matrix, is disclosed in U.S. Pat. No. 4,553,063, issued Nov. 12, 1985 to Geibig et al.
A variety of different fluorescent lamp types have been developed to meet different market demands. In addition to conventional tubular fluorescent lamps for office and commercial applications, compact fluorescent lamps have been developed as incandescent lamp replacements. Subminiature fluorescent lamps have found applications in displays and general illumination in limited spaces.
Different fluorescent lamps may operate under very different discharge conditions. The small size of subminiature fluorescent lamps may not allow for hot cathode operation, thus requiring efficient cold cathode emitters. The buffer gas pressure in subminiature fluorescent lamps is often on the order of 100 torr in order to limit electron loss to the lamp wall. By contrast, conventional fluorescent lamps typically utilize pressures on the order of 0.5-2.0 torr. Environmental concerns have necessitated the investigation of lamp fill materials other than mercury. In mercury-free fluorescent lamps, radiation is often produced by excimers of inert gases, such as neon, argon and xenon. In order to form excimers, a gas pressure of approximately 100 torr is required. In subminiature fluorescent lamps utilizing cold cathodes, the operating life may be limited by sputtering. In addition, current limitations may restrict light output. These trends have produced a need for improved cathode configurations.
The hollow cathode configurations disclosed in the prior art are not suitable for use in subminiature fluorescent lamps because of their relatively large sizes and because of the relatively high pressures utilized in subminiature fluorescent lamps.
Hollow cathodes have been studied in connection with other applications, such as excitation sources for gas lasers, ion sources, plasma jets, electron beams and plasma switches. In each case, a cathode with a single, relatively large opening, or hollow, has been studied at low (subtorr) pressure.